I. Rosas et al., THE ALGAL COMMUNITY AS AN INDICATOR OF THE TROPHIC STATUS OF LAKE PATZCUARO, MEXICO, Environmental pollution, 80(3), 1993, pp. 255-264
An evaluation of water quality and phytoplankton composition was carri
ed out in order to determine the trophic conditions of Lake Patzcuaro
(2035 m above sea level), a high altitude tropical lake. Temperatures
ranged from 15 to 23-degrees-C Total phosphorus and inorganic nitrogen
showed a seasonal variation; highest values coincided with the rainy
season (0.48 and 2.1 mg litre-1, respectively). Dissolved oxygen range
d from 2 to 7.9 mg litre-1 at the surface and from 0.6 to 7.3 mg litre
-1 on the bottom, the lowest values being found in shallow zones. Aver
age transparency varied from 0.62 to 1.4 m Secchi depth. Rainfall was
a primary factor in seasonal variability as it influenced both physica
l and biological conditions by contributing to the transport and depos
ition of silt, which mixed with sinking algal cells. The composition o
f the surface phytoplankton segregated along five major divisions comp
rising a total of 49 species. General seasonal patterns of dominance a
lternated in a sequence beginning with Bacillariophyta, through Chloro
phyta to Cyanophyta. Diatoms, the dominant group from February to earl
y June, included Melosira granulata, Stephanodiscus sp., Synedra sp. a
nd Fragilaria sp. During the rainy season (late June to September), Mi
crocystis aeruginosa, Oscillatoria sp., Anabaena sp., Merismopedia sp.
, Crucigenia cuadrata, Oocystis lacustris, Selenastrum gracile, Mallom
onas sp. and Tetraediella sp. were important. Melosira granulata was p
resent throughout the period of study. Spatial and temporal variabilit
y in both physical and biological conditions make it difficult to assi
gn a specific trophic state to Lake Patzcuaro. Nevertheless, analysis
of the algal community indicates a generally mesotrophic condition.